Metal ions such as stannous (Sn2+) and zinc ions are known to be effective anti-microbial agents. These metal ions provide anti-gingivitis and anti-plaque benefits and may also improve breath and reduce sensitivity. Stannous fluoride (SnF2) has been used in dentistry since the 1950's as a fluoride source to prevent dental caries.
However, it has been observed that aqueous solutions of stannous fluoride tend to precipitate water-insoluble stannic oxide (SnO2) upon aging. After a storage time of several months, such solutions may exhibit turbidity, which is believed to be due to the presence of very small crystals of stannic oxide dispersed in the solution, and the solutions may even form a sediment of stannic oxide at the bottom of the storage container. The precipitation of water-insoluble stannic oxide reduces the availability and therefore the therapeutic efficacy of the stannous ions in the oral care composition. It is believed that the precipitated water-insoluble stannic oxide has negligible or even no direct therapeutic effect.
The inclusion of chelating agents in aqueous solutions of stannous fluoride has been found to reduce the formation of this turbidity to some extent, but is also believed to impair the efficacy of the stannous ions.
Therefore, a need still exists for oral care compositions in which the bioavailability and therapeutic efficacy of the tin ions (stannous/stannic ions) is preserved, and which compositions do not exhibit turbidity upon aging.